Typically, pregrippers are used for the transfer of a sheet from a sheet feeder to a feed drum or an impression cylinder of a rotary printing machine. In order to ensure that the sheets are printed properly, the sheet feeder feeds the sheets to the pregripper with a predetermined alignment and the pregripper accurately transfers the aligned sheets to the feed drum or the impression cylinder in register.
A pregripper of this type is disclosed in DE-B 2,156,577, in which the pre-gripper essentially comprises a plurality of gripper bridges that are attached to holders which are releasably attached to a cylindrical tube. A problem with this design is that the pregripper has difficulty maintaining accurate in-register transfer of sheets when the printing machine is running at a high speed because the cylindrical tube tends to bend and twist. The bending and twisting of the cylindrical tube disrupts the transfer of the sheets causing them to be transferred out of register which results in lower quality printing. This problem is particularly acute when the printing operation requires both high printing machine speeds and relatively high register accuracy.
Another problem with this pregripper design is its complexity. The complex design makes it more difficult and time consuming for the pregripper to be assembled and disassembled. For example, in order to remove the cylindrical tube a driving lever must first be removed from the tube. The extra time required for assembly and disassembly of the pregripper increases both the cost and the production down-time associated with maintaining the pregripper.